<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style type="text/css">
        * {
            padding: 0;
            margin: 0;
        }

        li {
            list-style: none;
        }

        body {
            background: #f6f9fc;
            font-family: arial;
        }

        .calendar {
            width: 210px;
            margin: 0 auto;
            padding: 10px 10px 20px 20px;
            background: #eae9e9;
        }

        .calendar ul {
            width: 210px;
            overflow: hidden;
            padding-bottom: 10px;
        }

        .calendar li {
            float: left;
            width: 58px;
            height: 54px;
            margin: 10px 10px 0 0;
            border: 1px solid #fff;
            background: #424242;
            color: #fff;
            text-align: center;
            cursor: pointer;
        }

        .calendar li h2 {
            font-size: 20px;
            padding-top: 5px;
        }

        .calendar li p {
            font-size: 14px;
        }

        .calendar .active {
            border: 1px solid #424242;
            background: #fff;
            color: #e84a7e;
        }

        .calendar .active p {
            font-weight: bold;
        }

        .calendar .text {
            width: 178px;
            padding: 0 10px 10px;
            border: 1px solid #fff;
            padding-top: 10px;
            background: #f1f1f1;
            color: #555;
        }

        .calendar .text h2 {
            font-size: 14px;
            margin-bottom: 10px;
        }

        .calendar .text p {
            font-size: 12px;
            line-height: 18px;
        }
    </style>
</head>

<body>
    <div id="tab" class="calendar">
        <ul id="ul">
            <li class="active">
                <h2>1</h2>
                <p>JAN</p>
            </li>
            <li>
                <h2>2</h2>
                <p>FER</p>
            </li>
            <li>
                <h2>3</h2>
                <p>MAR</p>
            </li>
            <li>
                <h2>4</h2>
                <p>APR</p>
            </li>
            <li>
                <h2>5</h2>
                <p>MAY</p>
            </li>
            <li>
                <h2>6</h2>
                <p>JUN</p>
            </li>
            <li>
                <h2>7</h2>
                <p>JUL</p>
            </li>
            <li>
                <h2>8</h2>
                <p>AUG</p>
            </li>
            <li>
                <h2>9</h2>
                <p>SEP</p>
            </li>
            <li>
                <h2>10</h2>
                <p>OCT</p>
            </li>
            <li>
                <h2>11</h2>
                <p>NOV</p>
            </li>
            <li>
                <h2>12</h2>
                <p>DEC</p>
            </li>
        </ul>

        <div class="text" id="txt">
            <h2>1月活动</h2>
            <p>快过年了，大家可以商量着去哪玩吧～</p>
        </div>

    </div>
    <script>
        var month=document.getElementsByTagName("li");
        // console.log(month)
        for(var i=0;i<month.length;i++){
            month[i].flag=i+1;
            month[i].onclick=function(){
                for(var j=0;j<month.length;j++)
                {
                    month[j].className="";
                }
                this.className="active";
                switch(this.flag){
                    case 1:
                    document.querySelector("#txt>p").innerText="快期末考试了，你复习了吗";
                    break;
                    case 2:
                    document.querySelector("#txt>p").innerText="真的快过年了哦";
                    break;
                    case 3:
                    document.querySelector("#txt>p").innerText="快开学了，寒假作业写完了吗";
                    break;
                    case 4:
                    document.querySelector("#txt>p").innerText="这个月好像没有长假";
                    break;
                    case 5:
                    document.querySelector("#txt>p").innerText="51劳动节放假好开心呢";
                    break;
                    case 6:
                    document.querySelector("#txt>p").innerText="六一儿童节我也想有礼物";
                    break;
                    case 7:
                    document.querySelector("#txt>p").innerText="期末考试你准备好了吗";
                    break;
                    case 8:
                    document.querySelector("#txt>p").innerText="放暑假了";
                    break;
                    case 9:
                    document.querySelector("#txt>p").innerText="在家呆到发霉";
                    break;
                    case 10:
                    document.querySelector("#txt>p").innerText="10月份是个好月份";
                    break;
                    case 11:
                    document.querySelector("#txt>p").innerText="我快编不下去了";
                    break;
                    case 12:
                    document.querySelector("#txt>p").innerText="哪来这么多话说";
                    break;
                }
                document.querySelector("#txt>h2").innerText=this.flag+"月活动";
            }
        }
    </script>
</body>

</html>